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   Igneous rocks are formed on cooling of very hot and molten material  called magma .This hot material sometimes rises to the earth's surface through cracks caused  by earth quakes.The hot material then gets cooled after sometime and solidified .When molten rock break through and rises up to the surface of the earth as lava .The lava cools and crystallises rapidly .Extrusive igneous rock consists of  small crystals of dark coloured minerals called basalt rocks .